Suppose a tank in Oklahoma City leaks toxic chemicals. The area affected is a circle centered on the tank. When the spill is first noticed, the circle has a radius of 50 feet. A response team arrives 80 minutes later, by which time the affected area has grown to a circle with a radius of 70 feet. Required:
How rapidly was the radius of the spill changing in ft/min between the time the spill was noticed and the arrival of the response team?